feat: ModelManager accessor for bidirectional engines (2/N)

[GuanLuo]

Replace the process-scoped OnceLock<EchoBidirectionalEngine> registry in lib/llm/src/http/service/asr.rs with a ModelManager accessor keyed on model_name, parallel to chat / completions / embeddings.

When this lands, the install_engine / install_echo_engine install-time API retires and handle_socket calls state.manager().get_asr_engine(model_name) instead.

Prerequisite: a bidirectional-engine accessor must exist on ModelManager.